DESCRIPTION (applicant's abstract): This proposal is designed to enhance understanding of the full structure and function of the circadian visual system. The research effort is divided into four parts. The first concerns functional neuroanatomy, with emphasis on circadian system regulation by serotonin. These studies will determine whether serotonergic neurons in the median or dorsal raphe nuclei regulate circadian rhythm phase response or inhibition of light-induced FOS-IR in the SCN. Section two consists of experiments designed to evaluate the contribution of the subcortical visual system to circadian rhythm regulation. Experiments will evaluate the role of the intergeniculate leaflet and deep superior colliculus to determine their role as mediators of rhythm response to tonic light stimulation. Electrical stimulation of the deep layers of the superior colliculus will also be used to further determine how this area participates in circadian rhythm phase control and expression of FOS-IR in the SCN. The final section consists of basic neuroanatomy studies that will use anterograde and retrograde tract tracing and double label techniques to determine a) the patterns of IGL and VLG projections to the ventral and caudal midbrain; and b) the reciprocal serotonergic, GABAergic or other innervation between the median and dorsal raphe nuclei. We will also learn confocal microscopy and begin studying the extent to which there is retinal innervation of GABAergic SCN neurons that also contain light-induced FOS-IR neuronal nuclei. The studies will use immunohistochemical techniques, tracer application with iontophoresis, electrical brain stimulation to elicit rhythm phase shifts, and measurement of circadian rhythmicity of locomotor activity with computer-collected data records. The results will greatly expand understanding of the structure and function of the hamster circadian visual rhythm system.